This invention relates to a method of preparing poly(arylene ketones) and in particular to an electrophilic polymerization process for preparing such polymers.
Poly(arylene ketones), in particular, all para-linked poly(aryl ether ketones), possess many desirable properties, for example, high temperature stability, mechanical strength, and resistance towards common solvents. The preparation of poly(arylene ether ketones) by two different approaches has been described in the literature. The first approach is an electrophilic synthesis in which an aryl ketone linkage is formed. The second is a nucleophilic synthesis in which an aryl ether linkage is formed. This invention is directed to an improved electrophilic synthesis for preparing poly(arylene ketones), in particular para-linked poly(aryl ether ketones).
In such an electrophilic synthesis, the polymerization step involves the formation of an aryl ketone group from a carboxylic acid or acid derivative group and an aromatic compound containing an aromatic carbon bearing an activated hydrogen atom, i.e. a hydrogen atom displaceable under the electrophilic reaction conditions. The monomer system employed in the polymerization can be, for example, (a) a single aromatic compound containing both a carboxylic acid or acid derivative group as well as an activated hydrogen atom on an aromatic carbon for example, p-phenoxybenzoyl chloride; or (b) a two-component system of a dicarboxylic acid or acid derivative and an aromatic compound containing two activated hydrogen atoms, for example, 1,4-diphenoxybenzene and terephthaloyl chloride.
Electrophilic polymerization of this type is often referred to as Friedel-Crafts polymerization. Typically, such polymerizations are carried out in a reaction medium comprising the reactants(s), a catalyst, such as anhydrous aluminum trichloride, and solvent such as methylene chloride, carbon disulfide, nitromethane, nitrobenzene, or orthodichlorobenzene. Because the carbonyl groups of the reactant(s) and products complex with aluminum trichloride and thereby deactivate it, the aluminum trichloride catalyst is generally employed in an amount greater than one equivalent for each equivalent of carbonyl groups in the reaction medium. Other inorganic halides such as ferric chloride may be employed as the catalyst.
Such Friedel-Crafts polymerizations generally have produced an intractable reaction product difficult to remove from the reaction vessel and purify. Further, such processes have tended to produce polymer of undesirably low molecular weight and/or of poor thermal stability. The all para-linked poly(arylene ether ketones) have been particularly difficult to prepare under such Friedel-Crafts conditions. One factor that appears to contribute to the unsatisfactory results reported in the literature is that the para-linked polymers are more highly crystalline than the ortho, meta or mixed isomeric members of this polymer family and are therefore generally more insoluble in the reaction media typically used in such Friedel-Crafts reactions. This tends to result in the premature precipitation of the polymer in low molecular weight form. Another factor that may lead to these poor results is deactivation of the terminal aryloxy groups by complexation with aluminum chloride or alkylation of the terminal group which prevents further growth of the polymer chain. Also, side reactions, particularly at the ortho position of activated aromatic rings can result in a polymer that is branched and/or is more likely to cross-link at elevated temperatures such as those required for melt processing the polymer. It is generally recognized that in Friedel-Crafts reactions, ortho substitution of the polymer is more likely to occur if the reaction is conducted at elevated temperatures and/or for a relatively long reaction time. U.S. Pat. Nos. 3,065,205 to Bonner, U.S. Pat. Nos. 3,767,620 to Angelo et al., 3,516,966 to Berr, 3,791,890 to Gander et al., 4,008,203 to Jones and U.K. Pat. Nos. 971,227 and 1,086,021 both to Imperial Chemical Industries, Limited, disclose the preparation of poly(arylene ketones) by Friedel-Crafts polymerization and generally acknowledge some of the difficulties in producing tractable, melt-stable polymers. For example, Gander et al provide a method of producing the polymers in granular form by special treatment of the reaction mixture before gellation can occur and Angelo et al provide a method of treating the polymer to reduce undesired end groups which result from side reactions during polymerization and which cause thermal instability of the polymer.
To overcome the disadvantages encountered in producing poly(arylene ketones) by the above described Friedel-Crafts polymerization, it has been proposed to use boron trifluoride catalyst in anhydrous hydrogen fluoride. See for example, U.S. Pat. Nos. 3,441,538 to Marks, 3,442,857 to Thornton, 3,953,400 to Dahl, and 3,956,240 to Dahl et al. This general process has been used commercially to produce polymer of the desired high molecular weight and thermal stability. However, the use of boron trifluoride and hydrogen fluoride requires special techniques and equipment making this process difficult to practice on a commercial scale.
We have now discovered an improved process for the production of poly(arylene ketones) by an electrophilic synthesis which results in high molecular weight, thermally stable polymers using reaction media that are readily handled on a commercial scale. The process of this invention provides a high reaction rate which enables the reaction to be carried out at relatively low temperatures over a relatively short period of time. Further, the polymer is maintained in the reaction medium, for example in solution or in a reactive gel state, until high molecular weight polymer is obtained. Further, the polymer produced is essentially linear with little, if any, ortho substitution of the aromatic rings in the polymer backbone. Since the process of this invention maintains the polymer in solution or in a more tractable state, recovery and purification of the polymer is greatly facilitated.